The present invention generally relates to machining fluids for cooling and lubricating in hard turning machining applications. More specifically, the present invention relates to carbonated machining fluids which exhibit enhanced physicochemical properties to improve several aspects of the machining operation.
Large volumes of metalworking fluids are used in manufacturing industries each year for cooling and lubricating substrate work pieces and tools during machining operations. Cooling of either a tool or a workpiece at their interface involves both macroscopic and microscopic mechanisms. Macroscopically, heat elimination is a concern and typically involves directing a machining fluid onto the tool or work piece to remove heat in the form of thermal energy caused by friction between the tool and the workpiece. Microscopically, heat management involves reactive fluid constituents entering and reacting with tool/workpiece interface to reduce friction.
Increasing either the volumetric flow rate or the impingement pressure of a metalworking fluid at the tool/substrate interface is a conventional means for improving heat removal. Increasing the volumetric flow rate of the fluid near the boundary layer beneficially increases turbulence and heat removal in this region. This is achieved through exerting higher spray pressures of the coolant within the cutting zone, typically greater than 7 MPa (1000 psi). A variety of methods are suggested in the prior art for forming and delivering high pressure and high velocity metalworking fluids including, for example, the use of supercritical fluids. However, a number of challenges are associated with using such fluids that require relatively high pressures for proper delivery into the cutting zone. These include a need for expensive high pressure pumps and machining tool seal modifications to handle the increased pressures.
In addition to fluid pressure, a number of other factors can be considered to affect boundary layer fluid velocity. Metalworking fluid properties such as viscosity, surface tension, molecular size, and reactivity predominate at the cutting surface and within capillary interfaces. These physicochemical properties can be optimized to allow the metalworking fluid to penetrate the cutting zone more efficiently and to flow along surfaces at much higher velocities. For example, lowering viscosity and surface tension of the metalworking fluid is a way of improving boundary layer velocity to enhance heat extraction.
Conventional methods for lowering viscosity and surface tension include the use of base stock fluids having low viscosities and surface tension. For example, dense fluids such as high pressure liquids or supercritical carbon dioxide can be used as bulk metalworking fluids as a solvent, into which solute lubricant additives are added to provide the necessary reactive boundary layer constituents. However, like the high pressure metalworking fluid discussed above, these approaches are particularly expensive and complicated requiring specially designed high pressure plumbing in the machining tools. For example, the fluid seals of most machining tools cannot tolerate pressures greater than about 7 MPa (1000 psi).
Another technique includes the use of high velocity solid phase carbon dioxide composite coolant-lubricant sprays. Such composite carbon dioxide sprays resolve the high pressure limitations of dense fluids by delivering the beneficial chemistry of a dense fluid within a solid phase packet which is delivered in relatively low-pressure compressed air. Examples of these can be found in commonly owned U.S. Pat. No. 7,293,570 and U.S. application Ser. No. 11/301,466. However, in such composite sprays it can be difficult to maintain an appropriate solid-gas-liquid mixture through long distances through tortuous plumbing schemes found in many existing machining tools.
Finally, metalworking fluids of the prior art eventually accumulate microbial growth during use. Even new metalworking fluids, once dumped into a machining fluids sump, rapidly begin to degrade due to the microbial growth therein. Conventional methods involving metalworking fluid curb microbial growth by using, for example, biocides which have generally been found to be ineffective. New techniques are needed to control microbial growth.
The present predominant mode of cooling and lubricating during machining involves flooded applications of metalworking fluids. There therefore exists a need to improve the performance of these fluids. As such the present invention provides a means for boosting cooling and lubrication performance of conventional metalworking fluids and delivery systems, as well as providing a means for improving coolant longevity and quality.